A tool of the above-mentioned type is known, for example, from U.S. Pat. No. 5,351,797. The drive mechanism of such a tool is referred to as “having positive feed” in English or as “à avance mécanique” in French.
A single motor thus ensures, via the drive mechanism, that the spindle is driven in rotation about its axis and, at the same time, that it is advanced or returned by translation along its axis.
Since the translation and rotation drives of the spindle are connected mechanically, the advance of the spindle per revolution is constant. Thus, the variations in the speed of the motor have no effect on the rate of advance per revolution. The thickness of the swarf formed therefore remains constant and promotes the surface quality and the precision of the bores drilled by such a tool.
Although such a tool is found to be generally satisfactory, its use for drilling deep bores or materials renowned for being difficult, such as composite materials, may prove to be tricky.
For example, when a deep bore is drilled, the swarf accumulates in the flutes of the drill carried by the spindle until the motor of the tool is caused to stall. It is then necessary to proceed in several stages or to start fresh drilling cycles for the same bore to be drilled.
The drilling quality may also be degraded if the drill is repositioned incorrectly relative to the bore which has been started. The pressure of the swarf inside the bore also causes the deterioration of the surface thereof.
In order to be able to eliminate the swarf accumulated in the flutes of the drill, it would be possible to retract the spindle while continuing to drive it in rotation about its axis. Such a movement in fact corresponds to the return movement generally produced by the positive feed mechanism.
However, such a return would necessitate an interruption to the drilling cycle in progress, so that it would be necessary to start a fresh drilling cycle from the beginning.
The duration of the clearing operation carried out in that manner would be extremely long, leading to unacceptable drilling times.
DE 37 42 725 describes a machine tool which permits the rapid retraction of the spindle in the course of a machining operation. The return of the spindle is therefore ensured by a complicated system.
An object of the invention is to solve this problem by providing a tool of the type mentioned above which is simple and which permits the drilling of deep bores or materials renowned for being difficult in a reduced time.